The present invention is directed to the field of opto-electronic encoders with digital- or analogue-coded output or outputs.
In the related field, opto-electronic encoder devices essentially consist of an optical barrier, systematic removal of that barrier does encoding of a movement linked to the optical barrier. Documentation of pioneering work finds place in U.S. Pat. Nos. 2,537,427, 2,685,082 and 2,944,157. All of these patents have a rotatory disc with numerous radial apertures to allow light from a phototransmitter to pass. The thickness of the disc with coded scale was only governed by the strength of material with which it was fabricated. The thinnest possible discs became desirable, as the electrical light-producing methods were not very efficient. Their low efficiency and limited filament life marked incandescent lamps. Running them only at very low illumination levels could make them longer lasting. Initial light-emitting diodes (LEDs) too were not bright or efficient. That meant keeping the distance between the phototransmitter and the opto-electronic receiver as small as possible. Semiconductor manufacturers too started manufacturing such pairs housing a transmitter and a receiver spaced apart just by a fraction of a centimeter, in a package. This configuration made application of opto-electronics very easy for the encoder maker. U.S. Pat. Nos. 3,269,190, 3,304,434, 3,789,218 and 3,987,685 disclose the means to devise a multi-coordinate input device using two sets of disc encoders. Using another set of disc encoders to make it into a three-axis encoder could extend this scheme.
A simple three-axis encoder can be used to detect error signals and to make a toy robot remain upright. If the extended coordinate input device is used for this application, the ball has to have an eccentric center of gravity. Due to friction with the three rotating shafts rolling along the surface of the ball, the movement of the ball would not only be retarded, it could also fail to rotate sometimes. Friction-less data gathering solutions for comparative free rolling of the ball are disclosed in U.S. Pat. Nos. 5,831,553 and 6,686,584. U.S. Pat. No. 5,831,553 discloses a heavy ball as the central member with an eccentric center of gravity—suitable for the application presently discussed. The relative complexities involved in the implementation of both the schemes make them unsuitable for a cost-effective application. Furthermore, the use of a rolling magnetic element would cause ferromagnetic loose particles to attach to the rolling ball, and thus impede reliability.
For constructing a rotational single-axis encoder or input device, as noted earlier, use is made of an opto-electronic link placed axially on a circular optically coded disc. Though this construction has become an industry standard, there are two notable problems associated with this kind of axial mounting. The removal of the circular disc scale involves the removal of the optical transceiver pair; the minimum thickness of the whole encoder assembly together with the associated electronic components for processing the data seems to have reached a limit. There must be three layers in such an encoder—first, a printed circuit board (PCB) holding the phototransmitter and some electronics, next, the circular disc scale, and lastly, another PCB holding the opto-electronic receiver and the rest of the electronic components. By making use of thick-film technology, the overall thickness of this stack could be approximately 1.5 mm. Even a slight wobble in the disc scale due to anomalies would immediately damage the electronics on both the PCBs flanking the disc scale. Trying to increase the clearance on both the sides of the disc scale would definitely increase the thickness of the overall encoder assembly. There are also maintenance problems associated with this kind of construction. The accumulation of oil, moisture, or dirt on the scale goes unnoticed, until encoder failure takes place. Cleaning of the disc scale is possible with some care, but the cleaning of the optical transmitter and receiver active surfaces is very difficult. Replacement of a disc scale with a new one is also a complicated job, due to the basic axial positioning of the constituents of the optical transceiver pair on either flat side of the disc scale. Minute cracks in the body of a thermoplastic disc scale go unnoticed until mechanical failure occurs. U.S. Pat. No. 5,638,165 discloses a method of embedding optical fiber strands in a structure, and to gauge the thinning of the fibers at cracked positions. This method would be difficult to implement in a miniature mechanism like that of an opto-electronic encoder. The disc is constructed of transparent material, like glass or a transparent thermoplastic. A simple method which would give warning when small cracks appear in the circular disc would be of value, even with existing opto-electronic encoders.